FIG. 3 is a schematic view showing an example of a stacked structure of a top-emission organic light-emitting device. As shown in FIG. 3, in the organic light-emitting device (organic electroluminescence (EL) device), layers each made of an organic compound are stacked between a reflection electrode (anode) 21 for reflecting light which is provided on a support substrate 20 and a transparent electrode (cathode) “b” for transmitting light. For example, a hole transport layer 22, a light-emitting layer 23, an electron transport layer 24, and an electron injection layer 25 are stacked in mentioned order from the side of the reflection electrode.
For an organic light-emitting device of this type, there have been attempts to set an optical thickness of each of the layers in order to improve the light-extraction efficiency by an interference effect and to set a light-emitting region in order to prevent the reduction of internal quantum efficiency due to current flow. Japanese Patent Application Laid-open No. H07-78689 (page 2, lines 5 to 10) discloses that an emission peak of light generated in a light-emitting layer is made equal to an interference peak caused by an interference effect and used for light to pass through each of the stacked layers in order to improve the emission luminance of the organic light-emitting device. Japanese Patent Application Laid-open No. 2004-235015 (page. 2, lines 4 and 5) discloses that a light-emitting region of the light-emitting layer is set at a position away from an interface between the light-emitting layer and an adjacent layer by a predetermined distance in order to improve a lifetime characteristic. Japanese Patent Application Laid-open No. 2005-150043 (page 5, lines 47 to 50 and page 6, lines 6 to 11) describes that an optical distance from a reflection plane to an anode-side interface layer of an organic light-emitting layer is made shorter than one-fourth of a shortest wavelength of light emitted from the organic light-emitting layer in order to extract the light emitted from the light-emitting layer without changing an original color of the emitted light.
In the conventional organic light-emitting devices, the light-extraction efficiency is improved by the interference effect, and the internal quantum efficiency is stabilized by the light-emitting region control. However, the reduction of the light-extraction efficiency which is caused by change in a light-emitting region with the elapse of time as well as a measure to avoid the reduction are not known at all.
That is, even when the light-extraction efficiency is improved by the interference effect, a light-emitting device made of a conventionally known organic material has a problem that the internal quantum efficiency is gradually reduced by current flow, or the light-emitting region is gradually changed to reduce the light-extraction efficiency. The inventors of the present invention actually recognized a phenomenon that, even when the light-extraction efficiency of an organic light-emitting device using generally known tris[8-hydroxyquinolinate]aluminum (Alq3) as a material of a light-emitting layer was improved by optimizing the interference effect, the luminance of the device significantly was reduced by current flow.
As described above, although the organic light-emitting device in which the light-extraction efficiency is improved by the interference effect has an improved initial light emission efficiency, reduction in internal quantum efficiency and light-extraction efficiency simultaneously occur in the device. Therefore, there is a problem that the durability for stable light emission reduces. Then, when the device is used to produce a display apparatus, there is a problem that burn-in occurs.